1. Field of the Invention
The invention relates generally to a method for joining pipes, and more particularly to a method and a system for joining pipes comprised of low surface energy materials, such as HDPE (high density polyethylene), PEX (cross linked polyethylene), and PVDF (polyvinylidene difluoride). Low energy surface materials are generally to be referred to as olefinic materials within the scope of the disclosure and claims of this application.
2. Prior Art
A process called chemical fusion (employing an adhesive) is used to join olefinic plastic pipes comprised of low surface energy materials, such as HDPE (high density polyethylene), PEX (cross linked polyethylene), and PVDF (polyvinylidene difluoride). Olefinic plastic pipes such as those previously mentioned cannot be joined using the same technology to join, for instance, PVC, ABS and CPVC pipes. Pipes made of PVC or ABS or CPVC are typically joined using a solvent cement, where a solvent cement is a solution, emulsion or dispersion of the respective polymer in a volatile etching solvent. An example of a volatile etching solvent is a blend of THF, toluene, MEK. The solvent etches the surface of the pipe, and then dries leaving a coating of the cement and a partial fusion of the surfaces of the joined pipes. When olefinic pipes, such as those previously described, are attempted to be adhered in a similar fashion, the joint bond is unsatisfactory. The low surface energy olefinic plastics are resistant to solvent etching, and the residual adhesive cement does not satisfactorily adhere the two surfaces. Olefinic pipes are typically joined using mechanical means, such as threaded ferrule couplers, Victaulic couplings (e.g. clamps), or joined using heat fusion. With heat fusion, a current is induced in a wire coated with an appropriate olefinic material. The coated wire is wrapped around the pipe, between the inserting pipe section and the socket joint. The wire is inductively heated, which causes the olefinic coating to melt and fuse with the olefinic pipe.
A review of various alkyl borane adhesive systems as they relate to forming pipe joints follows.
U.S. Pat. No. 5,795,657 discloses a two part acrylic adhesive using an organo borane polyamine complex. Various organo borane polyamines, admixed with an oligomeric blend of acrylics, are tested against polyethylene, polyproplene, and Teflon.
In 3M's U.S. Pat. No. 6,383,655, they further address adhesives suitable for low energy surfaces, where the adhesives cure at room temperature. A low energy surface is defined as having a surface energy less than 45 mJ/m.sup.2, more typically less than 40-45 mJ/m.sup.2, or less than 35-40 mJ/m.sup.2. Cited as examples are polyethylene, polypropylene, acrylonitrile-butadiene-styrene, polyamide and fluorinated polymers such as polytetrafluoroethylene (Teflon™). Using the system described in U.S. Pat. No. 6,383,655, the adhesive is cured at room temperature. The substrates tested were polypropylene, polytetrafluoroethylene and high-density polyethylene. Over lap shears for the high-density polyethylene were up to 700 psi and 800 psi for the polypropylene. Over lap shear for fluorinated polyethylene was reported anywhere from 160 to 350 psi. The amines used in U.S. Pat. No. 6,383,655 were substantially shorter than the amines used in U.S. Pat. No. 5,795,657, suggesting that greater cross-linking probably occurred, and that the cohesive force of the adhesive is higher. Anticipated applications mentioned include adhesives, bonding materials, sealants, coatings, and injection molding resins. The disclosed adhesive systems may also be used as matrix resins in conjunction with glass, carbon, and metal fiber mats, such as those used in resin transfer molding operations. They also may be used in encapsulants, potting compounds, electrical components, printed circuit boards, and the like. There was no mention of the use of the adhesive for adhering pipe joints. Of interest is that when using the described adhesive system, they (3M) found it useful to add a thickener such as polybutyl methacrylate (col 11, line 56). This is in contrast to a potential issue for the instant invention, in that a low viscosity adhesive is desired.
U.S. Pat. No. 4,670,207 addresses a number of issues associated with joining olefinic pipe using a socket joint bonding method. According to the reported prior art, wherein the walls of socket and pipe are substantially straight, the insertion of the pipe into the socket produces a bead of adhesive that is expressed outward, and also produces a bead of adhesive that is forced inward. The inward bead can actually partially occlude the pipe, but more importantly is a potential source for contamination. A purported solution to the problem is using a socket and pipe insert where both have tapered walls. Reputedly, the tapering prevents expressing a bead of adhesive when the pipe is inserted into the socket. Reportedly, tapering the inner circumferential surface of the socket portion of the joint and the outer circumferential surface of the end portion of the pipe results in higher bonding pressure, and consequently a high bond strength can be obtained. While the tapering may help in aligning the pipes, in fact, it still would not solve the problem of keeping the end of the inserted pipe coaxial (concentric) with the socket joint, which is a feature of the invented unique pipe joint.
U.S. Pat. No. 5,449,207 is included to show that the concept of using adhesive delivery systems to pump the adhesive into the joint has previously been addressed. While not explicitly showing a similar type of coupling device, nor the use of a similar adhesive (alkyl borane adhesive), the patent does illustrate the use of an extruded adhesive.
U.S. Pat. No. 4,688,829 is included only to illustrate in a very general way a means of aligning the inserting pipe concentrically into the socket joint. In FIG. 3 of U.S. Pat. No. 4,688,829, there is a first annular ring 12 and a second annular ring 13, which align 7 the inserting pipe into 4, the socket joint.
In U.S. Pat. No. 6,436,224, Nishimura describes a silicon compound used in a solvent, typically toluene, to augment electro fusion (EF, which is another name for heat fusion). The silicon compound acts somewhat like a flux and a sealant. It is especially useful if the pipe has becomes oxidized or surfaces are contaminated with sand or some other loose material which prevents successful electro fusion.
Dow chemical has a pair of published patent applications (US Publication 2003/0047268A1 and 2003/0044553A1) on file that utilize alkyl borane adhesives. The applications discuss the adhesive composition, as they relate to methods to repair fuel tanks.
In Canadian Patent 1,028,376 to Silver et al., Silver teaches a pipe joint having a male joint member and a female joint member, where the end of the male member is tapered and the receiving female member necks down. The two are joined with what approximates a press fit at the male end. The mouth has a ferrule shaped flanged annular cap.
What is needed is a pipe joint suitable for joining pipe joints that hereto before have not been successfully joined using conventional pipe non-metallic pipe cements.
Further, what is needed is a pipe joint that can employ very high solids adhesives, as solvent based adhesive systems produce air emissions, which are progressively being regulated out of existence.